Difference between revisions of "CSC231 Homework 6"

From dftwiki3
Jump to: navigation, search
(→‎Debugging)
(→‎Debugging)
 
(3 intermediate revisions by the same user not shown)
Line 99: Line 99:
 
=Debugging=
 
=Debugging=
  
−
Lei Lei and I found a good way to debug the program with ddd.  The problem is that the program is supposed to start with the name of a bmp file on the command line, which makes it harder to debug with ddd.
+
Because the program assumes that you will provide a file on the command line (the name of the image file), if you want to debug your program with '''ddd''', then you need to proceed as follows:
−
 
+
* start '''ddd'''  
−
One solution is to modify the program slightly for debugging:
 
−
 
 
−
 
 
−
        section .data
 
−
errorMsg db    "Error accessing file!",0x0A
 
−
errorLen equ    $-errorMsg
 
−
'''filename db    "secret1.bmp", 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0'''
 
−
 
 
−
section .bss
 
−
 
 
−
''';filename resb  255 ; reserve 255 chars for filename'''
 
−
handle  resd 1 ; file descriptor
 
−
noRead resd 1 ; contains number of bytes read
 
−
lWidth resd 1 ; the line width rounded to the nearest
 
−
      ; dword boundary lWidth=(((width-1)/4)+1)*4
 
 
   
 
   
−
;;; ----------------------------------------------------------
+
    ddd hw6 &
−
;;; bmp buffer starts here
 
−
;;; ----------------------------------------------------------
 
−
bmpBuf equ $
 
−
type resw 1 ; header
 
−
size resd 1
 
−
resvd1 resw 1
 
−
resvd2 resw 1
 
−
offset resd 1
 
−
;;;
 
−
size2 resd 1 ; information header
 
−
width resd 1
 
−
height resd 1
 
−
planes resw 1
 
−
xbits resw 1
 
−
compr resd 1
 
−
imgsize resd 1
 
−
xres resd 1
 
−
yres resd 1
 
−
nocol resd 1
 
−
impcol resd 1
 
−
headLen equ $-bmpBuf ; length of header
 
−
 
−
pixelBuf resb MAXPIXEL*3 ; times 3 bytes (RGB)
 
−
 
 
−
 
−
;;; ---------------------------------------------------------
 
−
;;; code area
 
−
;;; ---------------------------------------------------------
 
−
section .text
 
−
global _start
 
  
−
_start:
+
* once ddd is open, put a '''breakpoint''' at the beginning of ''your'' code section, when the file has been read
−
;          '''jmp      start2'''
+
* Then using the top menu, click on '''Program''', then '''Run''', then enter the name of the bmp file you want to work with in the '''argument''' box.
  
−
;;; --- check that user entered more than 1 argument on command line ---
+
* Click '''Run'''.
−
pop ebx ; get # of arguments on command line
 
−
cmp ebx,1
 
−
jg process ; if user forgot file name, exit
 
−
dumpStr "Syntax: progName filename"
 
−
jmp exit
 
−
 
−
;;; --- get filename from command line ---
 
−
process:
 
−
pop ebx ; pointer to argv[0] (name of program)
 
−
pop ebx ; pointer to argv[1]
 
−
mov edi,filename ; pointer to filename
 
−
copy: mov al,[ebx] ; copy name of file from argv[1] to
 
−
mov [edi],al ; filename string, until \0 is copied
 
−
inc ebx
 
−
inc edi
 
−
cmp al,0
 
−
jne copy
 
  
−
'''start2:'''
+
You are now debugging your program after it has loaded the bmp file in question.
−
;;; --- open file ---
 
−
        openFile filename, [handle], errorMsg, errorLen
 
−
       
 
−
If you want to debug your program with ddd, just remove the semicolon in front of the '''jmp start2''' instruction, and the part of the code that gets the file name from the command line is removed from the code, and the file '''secret1.bmp''' is used as the default.
 
  
 
=Fun stuff=
 
=Fun stuff=
  
 
For your enjoyment, a [http://www.youtube.com/watch?v=-TKb7JrhJpk  movie]...
 
For your enjoyment, a [http://www.youtube.com/watch?v=-TKb7JrhJpk  movie]...

Latest revision as of 08:08, 30 October 2008

Steganography

--DT 22:26, 28 October 2008 (UTC)

Introduction

This homework is due on Wed. Oct 29th, at 11:59 p.m. + 1min.

It deals with steganography, and specifically the problem of decoding an images where the least significant bit of each byte is used to store one eighth of an ascii character.

An example will illustrate the concept. Assume that the first 16 bytes of a BMP image file are as follows: 

         A2 
         B2 
         C7 
         FF 
         00 
         20 
         31 
         30 

         AA 
         AA 
         AB 
         AB
         00 
         20 
         21 
         21

Let's rewrite the same 16 bytes, but this time we write next to each byte its least significant bit as a separate entity: 

         A2 0
         B2 0
         C7 1
         FF 1
         00 0
         20 0
         31 1
         30 0

         AA 0
         AA 0
         AB 1
         AB 1
         00 0
         20 0
         21 1
         21 1

You notice that when the byte contains an even number, the LSB is 0, and 1 otherwise.

Now, let's group these 16 least significant bits as 2 bytes. We get 00110010 00110011, or 0x32 0x33. We recognize here the ASCII codes for the character '2' and for the character '3'. We have in effect "hidden" the string "23" in the first 16 bytes of the image, and we have done so in such a way that the image still looks like an original when viewed by a human being. This is because the human eye cannot notice a 1-bit change in a color coded in 8 bits.

Of course, we assumed here that the MSB of the ASCII charcter is the first one that gets stored in the pixel bytes, but it could just as well be the LSB. You have to figure out how the secret message gets encoded into the pixel bytes...

Assignment

Your assignment is to write an "extractor" program that will take the name of a BMP file on the command line, load the image in memory, and extract from the least significant bits of the bytes forming the image a series of ASCII bytes that it will then print on the screen.

As is the convention for character strings in C, the secret message is terminated by a 0-byte, i.e. a byte whose 8 bits are 0 (as we did in class on Wed. Oct 22nd).

Use the following files as a skeleton for working with BMP files:

To open a bmp file and process it with makegreyBmp.asm, proceed as follows (assume the image file is called mountains.bmp): 

             nasm -f elf -F stabs makegreyBmp.asm
             ld -o makegreyBmp makegreyBmp.o

             makegreyBmp  mountains.bmp


Once the program returns to the Linux pompt, the mountains.bmp file will have been processed by your code, and hopefully the hidden contents will have been listed on the screen.

Test Files

Here are 4 images that contain secret information that your program should be able to decode:


Note that the first three images look absolutely identical, but all 3 contain different messages. I find it pretty remarkable that the secret message is stored in the first bytes of the first row of the bmp file, and for the Woodstock images, this first row is white, or appears white, but the bits are there, impossible to detect!

Requirements

You must use functions!

Submission

Once your program extracts a string that looks like English text, and no strange characters, you're done! Nothing else to submit except your program! Submit your program (called hw6.asm) as follows 

     submit hw6 hw6.asm

Debugging

Because the program assumes that you will provide a file on the command line (the name of the image file), if you want to debug your program with ddd, then you need to proceed as follows:

  • start ddd 
   ddd hw6 &
  • once ddd is open, put a breakpoint at the beginning of your code section, when the file has been read
  • Then using the top menu, click on Program, then Run, then enter the name of the bmp file you want to work with in the argument box.
  • Click Run.

You are now debugging your program after it has loaded the bmp file in question.

Fun stuff

For your enjoyment, a movie...

Retrieved from "http://dominiquethiebaut.com/dftwiki3/index.php?title=CSC231_Homework_6&oldid=1952"